This invention relates generally to large-diameter ducts, tubes, or cylindrical containers, and in particular to a process and apparatus for the continuous production of thermoplastic helical seam profile pipe.
Thermoplastic pipes of large diameter have found widespread use in storm and sanitary sewers, roadway drainage projects, and anyplace where gravity-flow conduit is useful. Pipes composed of high density polyethylene (HDPE) thermoplastic provide strength, durability, corrosion resistance, light weight, long service life, and cost effectiveness.
The continuous production of thermoplastic pipe and cylindrical containers has been accomplished in prior art using, among other methods, the helical seam technique. In that system, a strip of thermoplastic in a heated, plastic state is wound around a cylindrical surface or mandrel to produce a succession of turns of the strip which laterally abut or overlap each other. The abutting or overlapping turns are joined together, thereby forming a helically-wound pipe of any desired length.
In practice, helical pipes formed of strips of a tubular profile have demonstrated superior strength and rigidity to those formed of strips that are substantially flat. Pipes made from flat strips are often susceptible to being crushed when buried in the ground and exposed to radially-inward directed compression forces. By contrast, pipes made of wound hollow tubular profile strips exhibit good load distribution characteristics and can withstand larger forces.
Although superior in strength, the manufacture of tubular profile pipes has proven difficult. The steps of forming, winding, joining, and cooling are more complex when the continuous strip of hot thermoplastic is a hollow tube. Previous tubular profile designs have used thermal butt welds to join abutting turns of tube, whereby hot, un-cooled tubular strips are wound together. Abutting turns are naturally welded together on contact by the heat energy residual within the thermoplastic. A disadvantage is that a substantial portion of cooling must be accomplished after the tubular strips are wound. This necessitates compromises in the shape and design of the mandrel to accommodate air circulation or other cooling scheme.
Further, external seams or joints, at regions where abutting turns of pipe are joined can be uneven, resulting in an unsightly appearance. If the inconsistencies are severe, a pipe may have reduced strength when exposed to bending or compression loads.